DIODE LASER SPECTROSCOPY
GETTING STARTED
OVERVIEW of the INSTRUMENT
INITIAL SETUP
FIRST EXPLORATIONS
www.teachspin.com
Rev 2.0 11/09
I. Overview of the Instrument
(See the Apparatus Section 5 for details.)
A. The Laser
TeachSpin’s robust and reliable grating stabilized laser is both temperature and current
regulated. When the grating is in place, it provides optical feedback that retroreflects the laser
light to create an external cavity that stabilizes the laser to run at a controllable wavelength.
A piezo stack, mounted in the grating support, allows the grating position to be modulated by
an applied voltage. The laser temperature, laser current and piezo stack modulation are
determined by individual modules of the Laser Diode Controller.
A Plexiglas cover over the laser provides isolation from air currents and protects the
knobs used to adjust the angle of the grating from accidental changes. There are two holes in
the cover to allow the laser beam to exit undisturbed both with and without the diffraction
grating in place. (The grating can be removed to study the way the laser behaves without
grating stabilization.)
B. The Detectors
Your apparatus is supplied with three photodiode detectors. The detectors contain current
to voltage converters. The detector response is linear when the voltage output signal is
between 0 and -11.0 Volts so you want to make sure you are no where near the -11.0
saturation voltage. A switch on the back of the detector allows you to change the gain setting
from 10 MΩ to 333 Ω in ten steps. The detectors have separate signal and power cables.
Three DETECTOR POWER plugs are on the front panel of the controller. You can send the
detector signal directly to an oscilloscope or to the DETECTOR MODULE of the Controller.
C. The Absorption Cell Assembly
The absorption cell assembly consists on an outer glass cylinder, an insulation layer, a
heater assembly, a “cold-finger”, a thermocouple to monitor the temperature and the gas filled
Rb cell itself. The cold-finger is a small piece of metal that fits over a small protrusion on the
side of the cell. Because the metal is a good conductor and stays cooler than the cell, any
excess rubidium will condense in the protrusion, rather than on the windows of the cell. The
heater is both powered by and monitored from the controller.
D. The Magnetic Field Coils
The magnetic field coils are a Helmholtz pair which produces a uniform field at the
Rubidium cell. They are used in experiments such as Resonant Faraday Rotation and Zeeman
Splitting and must be powered by an external power supply. The Absorption Cell Assembly
in mounted at the center, even when they are not in use.
E. The Controller
While almost all functions of the apparatus are controlled by the modules on the front on
the Laser Diode Controller, the potentiometer used to set the laser temperature is on the back,
to prevent accidental changes. The laser temperature determines the lasing frequency and will
be set at the factory. The temperature should be touched only if, for some reason, a check of
the Laser Temperature Set Point indicates it has been altered or the diode itself is changed.
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Rev 2.0 11/09
The Modules – starting from the right
DETECTOR/LOW PASS/DC LEVEL: This module provides power for three detectors and offers
two detector inputs and a series of Monitor options. You can look at either detector or a
combined signal.
PIEZO CONTROLLER: This controls the piezo modulation, which determines the way the
angle of the grating is changed and thus the change or “sweep” of the laser frequency. It
includes a monitor output.
RAMP GENERATOR: This provides a bipolar variable amplitude and frequency triangle wave
which can be used, via the RAMP OUTPUT connection, to modulate either or both the piezo
stack and the laser current. The resulting changes in the grating angle and current produce the
variation or “sweep” of the laser frequency. The RAMP GENERATOR module can supply a wide
range of frequencies and amplitudes. The SYNC OUTPUT connection for the oscilloscope is
located in this module.
CELL TEMPERATURE: The cell temperature is both set and monitored through keys on the
LED display. It has been configured by TeachSpin. In case it is accidentally reset, see the
Apparatus section for detailed help.
CURRENT: The current module controls the current to the laser. It houses a modulation input
so that the current can be ramped along with the piezo stack and an attenuator to control the
degree of modulation.
MONITORS: This set of connectors and indicators, located on the lower part of the cell
temperature panel, provides a place to monitor, as a voltage, the set point temperature of the
laser as well as the actual temperature and current. The indicator lights indicate the
temperature of the laser in reference to the set point.
F. TV and Camera
The TV and camera will be used to observe both the light coming from the laser and the
Rb fluorescence in the vapor cell. While invisible to our eyes, the 780 nm light can be
detected by the camera and seen on the TV.
G. The Optics and Connectors
Your Diode Laser Spectroscopy system comes with a whole collection of bases supports,
mirrors, polarizers, neutral density filters and beam splitters which can be combined in a wide
variety of ways to do a wide range of experiments that is limited only by your imagination.
II. Initial Setup – What to do first
(This may take one or two hours.)
These instructions will help you set up and align your laser for the first time. When you have
carried out these detailed steps, your laser will be tuned to the Rubidium resonance lines.
Once aligned, it is unlikely that the laser will need any more than minimal tweaking.
A. Unpacking and Setting Up the Laser
1. The room used for your diode laser should be able to be closed to other users, first so that
you can dim the room lights, but most importantly so you can have absolute confidence
that no stray laser beams can escape and potentially cause harm to anyone.
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Rev 2.0 11/09
2. Unpack the various components from their shipping containers and place everything on a
table with plenty of room to work.
3. Place the Controller at one end of the optical breadboard. (we find it easier to put the
controller and laser on the right side of the board – the laser connection is behind the left
end of the controller.)
4. Mount the laser on one end of the breadboard so that the beam will go across the board.
(Before making placing the laser head on the board or making connections to it, ground
yourself to remove electrostatic voltages.)
5. Remove the protective plug from the laser head 9-pin D-connector. Connect the laser to
the controller using the 9-pin D-cable provided, which plugs into the back of the
controller.
6. Make sure the laser power switch (located on the left side of the controller, on the front) is
in the off position. Then plug in the laser controller power cord and turn on the main
power switch (located in the back, near the power cord).
B. Setting up the Absorption Cell Assembly – the cell takes a while to get to optimum
temperature so you want to have it heating up while you do other things.
1. Slide the Absorption Cell Assembly into the Magnetic Field Coils and secure it.
2. Place the assembly on the breadboard so that the laser beam will go through the cell. Put
it eight inches or so away so that you have room to work between it and the laser.
3. Connect both cables from the cell assembly to the back of the controller. You will notice
that the polarity of the banana plug heater wires does not matter. The polarity of the blue
thermocouple connector, however, does matter. It will only plug into the blue receptacle
one way.
4. When the power is turned on, the Cell Temperature controller (LED display on front
panel) will first reset and then display the cell temperature. In five or ten minutes the
cell temperature will be close to its factory established set-point temperature of 50 C.*
You may check and/or change the cell temperature set-point as follows:
a) Press the leftmost button on the cell temperature controller. It is marked by a
circular arrow. The temperature controller will read SP1.
b) Press the rightmost button on the cell temperature controller. The cell set-point
temperature (in degrees C) will now be displayed.
c) You can press the up/down arrow buttons to change the set-point. Start with a
temperature of 50 C.
d) Press the rightmost button. The display will read SP2
e) Press the leftmost button twice. The display will read RUN momentarily, then it will
read the cell temperature.
f) The cell temperature should read near the set-point after several minutes. You may
proceed with the next step before the final temperature is reached. The Cell
Temperature controller is not critical to operation of your diode laser. It merely
improves the signal strength by increasing the rubidium density in the cell. See
Theory section for a plot of Rb pressure versus temperature.
*
A starting temperature of 50 °C was chosen to give a nice strong absorption signal (about 90%). Once you
become familiar with the system you may want to work at a lower temperature.
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Rev 2.0 11/09
Trouble shooting: If the controller is not working as described please refer to the
Apparatus section of the manual under Cell Temperature Controller for how to
configure and set your controller. It is possible your controller became reset during
shipping or by a student (the ever-present scapegoat).
C. Starting up the Laser
Operating note: The diode laser frequency depends on temperature. If not set correctly, you
may not be able to get your laser to tune to the Rb resonance lines. The optimal
temperature was determined at TeachSpin and is recorded on the data sheet.
1. Check the Diode Laser temperature:
a) Use a voltmeter to read the TEMPERATURE SET-POINT in the MONITORS section
of the controller chassis. This voltage should equal the Temperature Set-point
recorded on the data sheet shipped with your laser. If it does not, adjust the 10-turn
potentiometer on the back of the chassis to obtain the correct set-point.
b) Make sure the LASER TEMPERATURE INDICATOR lights are both off. If either of
these is on, then the laser temperature has not yet reached its set-point temperature.
With a voltmeter connected to the LASER DIODE TEMPERATURE pin jacks, you
may monitor the laser temperature.
2. PUT ON SAFETY GOGGLES. Your laser typically runs with an output optical power of
10-30 mW, all concentrated into a narrow, intense beam. Staring directly at the Sun sends
about 1 mW into your eyes, and this is already enough to cause permanent eye
damage. To make matters worse, the laser light has a wavelength close to 780 nm, which
is nearly invisible. Practice proper laser safety – anyone that is in the room and can see
the laser, should wear safety goggles when the laser is on.
3. Set the laser CURRENT potentiometer fully counterclockwise (low current) then turn the
LASER POWER switch on.
D. Aligning the Laser
1. Locate the IR viewing card. The sensitive area of the IR card is a dull orange color. This
contains a polymer that absorbs UV light from the ambient lights in the room, especially
fluorescent lights. The polymer molecules are then excited into a metastable state, and
incident IR light from the laser can induce a transition that emits visible light. (Note the
IR card will not work well if the room lights are off for an extended period.) The IR card
allows you to “see”(actually locate) the laser beam even when you are wearing your
protective goggles, since the goggles do not block the visible light emitted by the polymer.
2. Hold the IR card at the laser output (the hole in the plastic cover of the laser) while you
turn up the laser CURRENT knob. You will need to turn the knob 3-4 turns before the
beam becomes detectable on the card.
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Rev 2.0 11/09
3. Set up the TV and Assemble the TV camera:
a) Put the TV monitor near the controller and set it up to display camera image.
b) Connect the power cable of the camera to the 12V power supply provided, and
connect the camera output cable to the TV monitor. You should see an image on the
monitor.
D
C ra
C me
a
C
c) Place the TV camera, mounted on an optical post, into a post holder. Then the camera
can conveniently be placed on the optical table with the laser and other optical
components.
Buisness Card
in Card Holder
Figure 1. External Cavity Alignment
Operating note: The camera lens can be focused over a broad range of working
distances, from infinity to as close as a few centimeters. The focus is adjusted by
turning the lens. Do NOT shine the laser beam directly into the TV camera, for this
may damage the CCD sensor.
4. Place a business card in the Neutral Density Filter holder and locate it so that it intercepts
the laser beam see Figure 1. Focus the TV camera on the card. Dim the room lights and
turn the laser current to zero. Now increase the current while watching the TV monitor.
You will see a light spot that becomes slightly brighter as you increase the current. Your
diode laser is below threshold, it is not lasing, but only acting as an LED. As you
continue to increase the current you will observe a sudden brightening of the beam spot
and the appearance of a speckle pattern characteristic of lasing.
Adjust the current so that the laser is just above threshold. You can measure the laser
current with a voltmeter. A diode current of 50 mA. will give 5.0 Volt output on the
LASER CURRENT in the MONITORS section. You can compare your measured value
with the threshold current recorded on your data sheet. A lower threshold current
represents better optical alignment. Do not be concerned if your threshold current is
slightly higher than that recorded on your data sheet. You will align the cavity in the next
few steps after which you can measure the threshold current again.
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Note: Your laser was shipped with the laser aligned. It many cases it will need little if
any adjustment. The following steps will allow you to check the alignment and optimize
it, if necessary.
5. Look at the laser head itself. You will see two knobs protected by the plastic cover. The
upper or top knob controls vertical alignment. The lower or side knob provides a
wavelength selective horizontal alignment. Before beginning your alignment, it is
important that you have read the first section of this manual Diode Laser Physics. It will
be much easier to follow the procedure if you have some idea of the physics behind these
adjustments. This may be the most difficult procedure you will need to follow in this
experiment. For the uninitiated it is very easy to totally misalign the laser, which can be
both frustrating and time consuming. If you are not familiar with diode laser adjustment,
we ask that you follow each step closely. If you have trouble or do not observe what is
described in a given step, do not go on to the next step! We have tried to anticipate
possible problems and direct you to the solution. We also do not want to make you overly
timid by this statement. Alignment of the external laser cavity is something that any
experimental physicist can accomplish. You will need to become facile in the alignment,
not only because your students may misalign the cavity, but also because eventually your
diode will burn out, and you will have to replace it. This will involve an alignment of the
cavity, starting from scratch.
Figure 2: Picture showing TOP and SIDE Knobs used to align grating. Allen wrench is
shown in Side Knob.
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6. Vertical Alignment: Remove the plastic cover from the laser and set the current so that
the laser is just above threshold. Adjust the TOP knob while watching the laser spot on
the card using the TV camera. Keep track of where this knob started, and DO NOT
TURN THE KNOB MORE THAN ONE HALF TURN. You may find it useful to use the
5/64” Allen wrench placed in the back of the knob as a position indicator. It is also easier
to make small adjustments of the knob by using the long arm of the Allen wrench as a
lever.
You should see the laser spot change in intensity as the TOP knob is turned. If you rotate
the TOP knob very slowly, you will notice that the bright “region” actually changes from
bright to dim. These are modes of the laser. You should be able to distinguish six to ten of
these modes, with fewer modes when the current is just above threshold. You are seeing
different longitudinal modes in the external cavity defined by the grating and back facet of
the diode. As you turn the top knob you are not only changing the grating angle but also
the external cavity length. You have changed the cavity length by one half wave length
when you move from one bright peak to the next.
You will need to have to set the
current just above threshold to see this clearly. This may involve a few iterations of
setting the TOP knob to give an intensity maximum and then adjusting the laser current.
Figure 3, at the right, shows an
oscilloscope trace of the intensity of the
laser as the TOP knob is adjusted. It
will give you an idea of how the
brightness of the spots you are seeing is
varying. It is hard to tell the middle
ones apart. For best alignment leave
the laser in the middle of this vertical
mode pattern as best you can. It is not
necessary to sit right on one of the
mode maxima, but only near the center
of the mode pattern. The correct mode
maximum will be set later with the side horizontal adjustment knob and piezo voltage.
Note that finger pressure on the knob also changes the grating alignment, so remove
your fingers often during this adjustment. If you find it difficult to turn the knob with
a light touch, then you can use the Allen wrench placed in the back of the knob as a
lever for adjustment.
It is not critical for operation of your laser that you achieve near perfect vertical
alignment of the grating. You will get adequate laser performance by simply turning
the TOP knob to the intensity maximum. However, it has been found that the better
the alignment the better the operation of the laser. Better operation being defined as
wider mode-hop-free scans.
If you are not able to see any change in laser intensity as you adjust the Top knob then
STOP! Do not continue. Most likely both the SIDE and TOP knobs have both been
moved by accident or during shipping. Please refer to “Aligning the external cavity”
in the Apparatus section of the manual.
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Rev 2.0 11/09
Operating note : The TOP and SIDE knobs are used to align the grating with respect
to the diode. The lines on the grating run vertically. Figure 2 shows the diode laser
with the cover off and the 5/64” Allen wrench placed in the SIDE knob. The first
order diffraction from the grating is directed back into the diode. The zero order
reflection from the grating is the light you observe leaving the laser. The TOP knob
rotates the grating about an axis that is parallel to the table top. Turning the TOP
knob changes the vertical angle of the light diffracted from the grating. But to first
order it does not change the wavelength of the light that is diffracted back into the
laser. The SIDE knob rotates the grating about an axis that is perpendicular to the
table top. Turning the SIDE knob does changes the wavelength of the light that is
diffracted back into the diode.
E. Setting up to Observe Rubidium Fluorescence
1. Remove the index card and position the Rubidium Absorption Cell Assembly so that the
laser beam passes through the center of the cell. You may use the IR viewing card to trace
the path of the beam.
CCD
Camera
Side
Hole
Rb
Cell
Cell Heater
ND Filter Holder
with Viewing Screen
(Beam Block)
Field
Coils
Figure 4. Setup for Observing Rubidium Florescence
2. Point the camera so it looks into the Rb cell from the Side Hole in the cell heater. If you
place the camera up on the base of the cell holder you can position the camera so that it
abuts the glass holder surrounding the Rb cell. It may also be helpful to dim the room
lights since you will be looking for the fluorescence light emitted by the Rb atoms.
3. Set up the two channel oscilloscope that you will use for these experiments. Run a BNC
cable from the RAMP OUTPUT of the RAMP GENERATOR module to an oscilloscope. Run a
second cable from the RAMP GENERATOR SYNC. OUTPUT to the ‘scope trigger. Observe the
output on the ‘scope as you adjust the RAMP GENERATOR settings.
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4. Use The RAMP GENERATOR and PIEZO CONTROLLER to Set the Frequency Sweep
a. Turn the ramp amplitude down and connect the RAMP OUTPUT from the oscilloscope to
the modulation input connection on the PIEZO CONTROLLER MODULE. This is a good
place to use one of the short BNC cables that came with the system.
b. Connect the MONITOR OUTPUT of the PIEZO to Channel 1 of the oscilloscope. Turn the
piezo OUTPUT OFFSET knob to zero. (The OUTPUT OFFSET changes the DC
level of the monitor output. It does not change the voltage applied to the piezo stack.
This control is used when locking the laser to an absorption feature and is not needed
here.)
c. Set the ramp generator frequency to about 10 Hz. Turn the piezo ATTENUATOR knob
to one (1). Set the ramp generator AMPLITUDE knob to ten (10) and use the DC
OFFSET knob of piezo controller to produce a large-amplitude triangle wave that is
not clipped at the top or bottom. The piezo MONITOR OUTPUT should have a
signal that runs from about 3 volts to about 8 volts.
Operating note: The PIEZO CONTROLLER drives a small piezoelectric stack that moves
the optical feedback grating. This scans the laser frequency (see the Diode Laser Physics
section of this manual for more on how this works).
CURRENT
CELL TEMPERATURE
RAMP GENERATOR PIEZO CONTROLLER
DC OFFSET
FREQUENCY RANGE
.5
.4
.3
.2
100 mA
Full Scale
.4
Laser
Temperature
Indicator
.6
.2
.8
0
MONITORS
Laser Diode
Temperature
1
.7
.8
.9
.1
1
1
0.1
10
0.8
100
1k
0.4
30us
10us
min.
1.6
0
Multiplier
2
6
8
2
3
1
.4
2
4
.6
2
.8
0
10
AMPLITUDE
Temperature
Set Point
0
5
OFFSET
RAMP
SYNC.
OUTPUT
OUTPUT
0
1
ATTENUATOR
MODULATION
+
10ms
10
DC OFFSET
5 10 20
50
1
3
1
_
Laser Current
INPUT
2
30
GAIN
_
0
5
OUTPUT
OFFSET
MONITOR
2
6
4
8
0
10
6
2
8
0
10
LASER DIODE CONT
(Laser Saftey Goggles Require
BALANCE
OUTPUT
DETECTOR INPUTS
To Oscilloscope Chan. 1
To Oscilloscope Trigger
Figure 5: Modules showing connections for setting the frequency sweep.
3-9
20
4
Below Set Pt.
INPUT
5
15
100
GAIN
+
.2
MONITOR
1
FINE
COARSE
100V Full Scale
DC LEVEL
30ms
0.1s
TIME CONSTANT
4
Above Set Pt.
0.3ms 1ms
0.1ms
3ms
MONITOR
1.2
Reset
4
LOW PASS
10k
0.01
Frequency
(Hz.)
2
ATTENUATOR
MODULATION
.6
DETECTOR
DETECTOR POWER
Rev 2.0 11/09
F. Actually Finding the Rb Fluorescence, Initial Horizontal Adjustment
1. Set the laser current to the value listed on your data sheet. You will need to connect a
voltmeter to the LASER CURRENT MONITOR to accurately set the current. If the horizontal
grating position has not been changed much during shipping or because of accidental
adjustment, then you will see a flashing streak of light within the cell on the TV monitor.
This is rubidium fluorescence. Atoms of Rb in the cell, absorb laser light at the atomic
resonance frequency and re-emitting it in all directions. If you do not see any
fluorescence, do not despair. You only need to make a slight adjustment of the SIDE
knob.
2. Put the 5/64” allen wrench (hex key) in the back of the SIDE knob and use it as a rotation
marker. Remember the starting position of the wrench; you could even draw a little
picture in your lab book. While you observe the TV screen looking for the fluorescence
flash, slowly rotate the SIDE knob first one way and then the other. You should not need
to rotate it more than one half turn in either direction.
3. If still no fluorescence is observed, then return the SIDE knob to the starting position, and
adjust the current in 3mA increments (about 1/3 of a turn) both above and below the Laser
current recorded on you data sheet. At each new current setting rotate the SIDE knob
again, so that you don’t lose your position, always return the knob to its starting position
before changing the laser current. If you do NOT see any fluorescence, first repeat the
above steps again, doing them with care. You might have someone else go through the
steps as well. It’s easy to miss some detail and thus not observe fluorescence. In
particular check the laser temperature, the vertical alignment, and make sure you are
sweeping the piezo.*
4. If you still see no florescence then you can try making bigger excursions in the grating
angle with the side knob (plus and minus one whole turn). It may be that the Cavity
became grossly misaligned during shipping, refer to section A4-2. in the appendix for
details on aligning the external cavity.
5. Once you see the florescence flash move the SIDE knob so that the florescence is always
visible. Now adjust the laser current to make the florescence as bright as possible.
G. Observing the Absorption Spectrum Using a Photodiode Detector
1. Connect a Photodiode Detector (PD) cable to the DETECTOR POWER output of the laser
controller, and connect the Photodiode Detector output BNC to Channel two (2) of the
oscilloscope. Set the Channel two (2) input coupling to DC, the gain to 5 Volts/div, and
the vertical position so that ground is in the middle of the oscilloscope display. The signal
from the Photodiode Detector is negative and saturates at about -11.0 volts. If you are
uncomfortable observing a negative going signal, you can always use the invert
function on your ‘scope.
*
You can check that the piezo is actually moving by doing the following. With the Ramp generator
connected to the Piezo modulation input, turn the AMPLITUDE of Ramp to zero, change the ramp frequency to
about 3 kHz. And then increase the AMPLITUDE. You should be able to hear the piezo vibrate. WARNING:
Do not leave the piezo running at high frequency and amplitude for a long time. It will cause heating and
damage to the piezo.
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Rev 2.0 11/09
2. Put the Photodiode Detector in place to intercept the laser beam coming through the Rb
cell. You can move the PD for alignment. You will have to adjust the Gain on the back
of the PD. Make sure that the beam is hitting the sensor and bolt the photodetector down.
Operating note: In the present configuration there is a very high intensity beam (power
per unit area) going through the Rb cell. This much power “saturates” the transition,
resulting in very little total absorption of the beam.
3. Attenuate the Signal Reaching the Detector
a) Assemble the glass neutral density filter in a fixed mirror holder and place it in a post
holder. Please refer to the Optics section in the Apparatus Chapter of the manual if
you are unfamiliar with putting optical components into mounts.
b) Place the attenuator between the laser and the Rb cell (not between the cell and
photodiode). Adjust the PD Gain so you can observe something on the ‘scope
showing that light is hitting the PD. For the best performance you want the PD Gain
to be as high as possible without saturating the PD. This keeps the noise from the PD
at a minimum. The PD gain changes in 1,3,10 steps, a signal level of 2 to 6 volts is
reasonable. Block the beam with your hand to convince yourself that the PD is
detecting the transmitted laser light.
You should see a ‘scope signal that looks something like this:
Figure 6: Upper trace (Channel 1) shows piezo monitor signal.
Lower trace (channel 2) shows Detector output. Note that signal is “negative
going” so absorption features appear as spikes.
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Rev 2.0 11/09
4. Interpreting the Oscilloscope Signals
a) The upper trace shown in Figure 6 is the piezo monitor, which shows the voltage on
the piezoelectric stack as a function of time. The lower trace is from the photodiode,
showing absorption dips due to the Rb vapor. Because we have a negative going
signal, these appear as spikes. If the laser scanned “perfectly” in frequency (that is no
mode hopping), you would see just some fraction of the Rb absorption spectrum. The
energy levels of 85Rb and 87Rb and the Doppler broaden spectrum are show below:
Figure 7: Energy level Diagrams
Figure 8: Doppler broadened spectrum
b) The absorption dips in your trace are interrupted by various “mode hops” – when the
laser frequency jumps suddenly. Refer to the Diode Laser Physics section for a
discussion of mode hops. Observe how the signal changes when you vary the laser
current and the piezo drive parameters. Please explore the parameter space.
H. Horizontal Modes, Final Horizontal Adjustment
1. Adjust the laser current and piezo voltage so that a “nice” absorption spectrum is centered
on the oscilloscope. This takes a little practice. As with the vertical adjustment, there are
also horizontal modes. These modes are slightly different, in that turning the horizontal
move through two or three of these modes by changing the piezo DC LEVEL voltage.
a) Place the 5/64” allen wrench (hex key) in the back of the SIDE knob with the long arm
of the allen wrench sticking out at about a 45° angle. (See picture at step 15). You
will use the allen wrench as a lever to gently move through the horizontal modes.
b) Watch the oscilloscope display as you gently push on the end of the allen wrench.
You should be able to identify six to eight modes in which the Rb absorption is still
visible on the oscilloscope. You want to set the Side knob in the middle of this mode
pattern.
c) You might notice that the modes at the ends have a shorter and more erratic scan over
the Rb absorption. You do not need to make an exact adjustment with the Side knob
as the Piezo DC OFFSET voltage can be used to fine tune to the mode. With proper
alignment and laser current adjustment you should be able to set a scan that covers the
first three lines in the absorption spectrum (87b, 85b, and 85a as shown in Figure 9).
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Rev 2.0 11/09
Figure 9: Scan showing first three absorption lines
2. You may notice a few “extra” features at the ends of a scan right before a mode hop.
These feature look like (and are) replicas of the strong 85b and 87b absorptions and
appear near where you would expect to find the 87a absorption. The “extra” features are
associated with relaxation oscillations in the diode laser. (See Diode Laser Physics
Section). By reducing the laser current and adjusting the Piezo DC LEVEL, you should
be able to get a nice scan showing the 85a and 87a features. This is shown in Figure 10.
Figure 10: Scan improved by adjustments to laser current and piezo DC level.
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Rev 2.0 11/09
I. Using Simultaneous Current and Piezo Modulation to produce a larger scan range
without mode hops. (See the Diode Laser Physics section for an explanation.)
1. Set the laser CURRENT ATTENUATOR knob to zero. Attach the BNC splitter “F” connector
to the RAMP OUTPUT on the RAMP GENERATOR. Plug one BNC from the RAMP OUTPUT to
the MODULATION INPUT of the PIEZO CONTROLLER, and the second BNC from RAMP
OUTPUT to the CURRENT MODULATION INPUT.
2. Turn the ramp generator amplitude up to maximum, and watch what happens when you
turn up the current attenuator knob. With some tweaking you should be able to produce a
full trace over the Rb spectrum. The oscilloscope invert function has been used to show
the trace in what “looks” more like an absorption spectrum in the Figure 11. Note the
correspondence to the expected atomic Rb spectrum shown in Figure 8
Figure 11: Expanded Scan Showing Four Absorption Lines
Operating point: The depth of the lines depends on the length of the Rb cell and the
atomic density, the latter depending on cell temperature. You can explore this by
changing the cell temperature.
3. You can see in the above that the background intensity changes considerably with the
scan. This is because you are now scanning the laser intensity (via the current) together
with the laser frequency (via the piezo). You can correct for this effect in a number of
ways. One way is to digitally record a spectrum with the cell in place, and then record a
second spectrum with the cell removed, and finally divide the two signals in software.
This has the advantage that only a single photodetector is needed, but the disadvantage
that the two traces are not recorded simultaneously. Another way to accomplish this is by
using a second photodiode, as in the following layout.
3 - 14
Rev 2.0 11/09
Detector 2
to (-) INPUT
CCD
Camera
Photodiode
Detector
ND Filter Holder
with Gelatin Filters
Rb
/5
50
Photodiode
Detector
Side
Hole
0
Cell
Cell Heater
S.
B.
Detector 1
to (+) INPUT
Glass
ND Filter
Field
Coils
Figure 12: Apparatus set up to use two detectors
J. Using Two Photodiode Detectors to Compare a Beam directly from the Laser to one
that has passed through rubidium vapor
1. You will need to place the 50/50 Beam splitter in a mirror mount. Please refer to the
Optics section in the Apparatus Chapter of the manual if you are unfamiliar with putting
optical components into mounts.
2. With this experimental configuration you will detect two simultaneous signals, one with
and one without the Rb absorption, and then subtract the spectra. You will use the
Detector electronics on the Laser Diode Controller.
To Oscilloscope Chan. 1
DANGER CURRENT
CELL TEMPERATURE
RAMP GENERATOR PIEZO CONTROLLER
Laser Saftey
Goggles
Required for
Everyone in
Lab
.3
.2
100 mA
Full Scale
.4
.8
0
MONITORS
Laser
Temperature
Indicator
.6
.2
ON
Laser Diode
Temperature
1
MODULATION
.6
.7
.8
.9
.1
1
10
0.8
100
1k
30us
10us
min.
1.6
10k
0.01
0
Multiplier
2
6
8
0
10
AMPLITUDE
Temperature
Set Point
0.3ms 1ms
0.1ms
3ms
1.2
0.4
2
3
1
.4
4
.6
2
1
3
.8
1
0
5
OFFSET
RAMP
SYNC.
OUTPUT
OUTPUT
0
1
ATTENUATOR
MODULATION
15
10
DC OFFSET
5 10 20
2
50
5
_
25
30
GAIN
Laser Current
INPUT
0
5
OUTPUT
OFFSET
MONITOR
OUTPUT
BNC "F"
To Oscilloscope Trigger
GAIN
2
6
4
8
0
10
6
2
8
0
10
LASER DIODE CONTROLLER
(Laser Saftey Goggles Required)
BALANCE
DETECTOR INPUTS
From
PD 2
DETECTOR POWER
From
PD 1
Figure 13: Controller Modules showing connections for using two photodiode detectors
3. Connect the BNC from the Photodiode Detector 1 to the right most (-) DETECTOR INPUT.
(This will invert the signal from PD 1 so that absorptions will show as dips.) Set the
BALANCE knob above the (-) INPUT to 1.0 (fully CW). Set the GAIN to 1. Connect a BNC
Cable from the MONITOR connector above the gain triangle to Channel 1 on the ‘scope.
3 - 15
OUTPUT
4
Below Set Pt.
INPUT
20
100
+
.2
+
10ms
1
FINE
COARSE
100V Full Scale
DC LEVEL
MONITOR
30ms
0.1s
TIME CONSTANT
Reset
4
LOW PASS
MONITOR
4
Above Set Pt.
OFF
1
0.1
Frequency
(Hz.)
2
ATTENUATOR
LASER
POWER
.5
.4
Class 3B Laser
780 nm.
80mW CW
DETECTOR
DC OFFSET
FREQUENCY RANGE
Rev 2.0 11/09
Change the BALANCE knob and observe the effect on the ‘scope. Position a second
Photodiode Detector to intercept the beam that has been split off by the Beam Splitter.
Connect the power cable of the detector to one of the open ports. Connect the BNC from
Photodiode Detector 2 to the (+) or left most DETECTOR INPUT. Set the BALANCE knob
above the (+) INPUT to 1.0 (fully CW) and turn BALANCE knob above the (-) INPUT to 0.
Adjust the Gain on the back of Photodiode Detector 2 for a “good” (2-6 volts) level signal
on the ‘scope and position the photodiode for a maximum signal. Now increase the
BALANCE knob above the (-) INPUT to 1.0 and adjust the BALANCE to get a spectrum like
the one is Figure 14.
Figure 14: Trace for Channel 1 only showing the combined signal from the detectors.
Subtracting the signals removes the effect of “ramping” the current. The beams
reaching both detectors are varying at the same rate and the BALANCE controls are
used compensate for any difference in intensity.
The trace shows an excellent correspondence to the expected spectrum, with all four Rb
absorption dips on a flat background. Note, however, that the subtraction technique does not
immediately give an absolute measurement of absorption, while the digital method does.
Operating note: You should always be wary that zero light on a photodiode may not
correspond to zero voltage output. You can check this by simply blocking the beam and
noting the voltage
3 - 16
Rev 2.0 11/09
K. All Finished
At this point the initial alignment is complete, and you are ready to move on to the more
sophisticated spectroscopy experiments. You may need to realign the grating feedback
from time to time, following the above procedures. If not disturbed, however, the
alignment will likely be stable for months.
L. Shutting Down
If you are not using the laser for a few hours for some reason, you can you can leave the
controller on. Then the diode laser and the Rb cell will stay at their operating
temperatures and be ready to go when you need it.
BUT TURN THE LASER CURRENT OFF. You should turn the laser current down, and
the laser power switch off, whenever you leave the lab. This is a safety precaution, plus it
will prolong the life of your laser. With use the diode laser will eventually burn out and
need to be replaced, so leave the laser itself off when not in use.
It’s okay to leave the ramp generator and piezo controller on and running at whatever
setting you wish (for examples, the settings determined above). You can also leave the Rb
cell temperature at whatever setting you wish. Then these will be set up when you want to
use the laser – just turn on the laser power switch and turn up the current. After the laser
warms up briefly, you should have essentially the same spectrum you had when you
turned the laser current off.
We do not recommend leaving the controller on overnight and unattended even if the
laser current has been turned off.
3 - 17
Rev 2.0 11/09
III. Observing Saturated Absorption
A. The Optical Plan
There are countless ways in which the optics could be configured to do observe the Saturated
Absorptions Spectrum (SAS of Rubidium. A complete diagram of the configuration we will
guide you in building is shown below in Figure 1. (A different layout is used in the lab notes
from Caltech which are at the end of this manual.)
Detector 2
to (+) INPUT
Detector 1
to (-) INPUT
Photodiode
Detector
Photodiode
Detector
CCD
Camera
M
BS
irr
0
or
2
/5
50
Rb Cell
/9
10
M
0
irr
1
BS
or
Glass
ND Filter
Figure 1. Complete SAS setup
B. Some Basics Before We Begin
1. Keep the beam height above the table constant as you bounce the beam off the mirrors.
Since the center of the absorption cell and the laser are 4” (10 cm) above the table top, the
beam should be there also. You can use the viewing card to check the beam height.
Place the viewing card in the neutral density beam holder so that the marked line matches
the top of the holder. Now, set the height so that the top edge of the holder, and thus the
center of the viewing card, is 4 inches above the tabletop.
3 - 18
Rev 2.0 11/09
2. When placing optics, try to start with the beam centered in the optic. This gives you
maximum adjustment range before the beam “walks off” the end of the optic and you have
to reposition the mount.
3. When using the optical mounts to hold beam splitters, observe that there are two possible
configurations of the mount. When looking at the mount from above, the upper
adjustment screw can be placed on the right or the left. If placed on the wrong side, the
support for the upper adjustment screw will block the transmitted beam. The upper
screws are shown with a blackened edge in the figures below. To change orientations,
you must remove the mount from the post and use the orthogonal mounting hole.
4. Spend a bit of time planning your optical layout before you start.
3 - 19
Rev 2.0 11/09
C. Placing the Components
Now that you have completed the Initial Setup and have observed the Doppler broadened
absorption spectrum of Rubidium you are ready to look for saturated absorption.
1. Make sure you have two mounted mirrors, a 10/90 and a 50/50 beamsplitter assembled.
2. Reconfigure the apparatus you have been using into the layout shown in Figure 2. (This is
only part of the complete SAS setup. We’ll add the rest later.)
BE SURE TO HAVE A BEAM BLOCK IN PLACE AS SHOWN
Detector 2
to (+) INPUT
Detector 1
to (-) INPUT
Photodiode
Detector
Rb Cell
CCD
Camera
Photodiode
Detector
/9
10
0
Beam
Block
BS
Glass
ND Filter
Figure 2. Start of SAS setup with 1O wedged beam splitter in place
We have used the 1O wedged beam splitter which yields two reflected beams, one from each
face. The small angle of the wedge causes the beams to diverge slowly so that both beams
can travel through the cell to the two photodetectors. The second photodetector (PD) is not
needed to “see” the SAS. It is used in the final electronic subtraction to remove the
absorptive background signal. If you do not intend to use this electronics “trick,” you can
leave the second detector out of the setup. Position the PD’s to maximize the signal level
from each.
3 - 20
Rev 2.0 11/09
Monitoring the output of the Detectors, you should observe the now familiar Rb absorption
spectrum on your ‘scope.
3. Now, add the two turning mirrors to the setup, as shown in Figure 3. Move the Beam
Blocker to the new location shown.
Detector 2
to (+) INPUT
Detector 1
to (-) INPUT
Photodiode
Detector
irr
or
2
Photodiode
Detector
Rb Cell
CCD
Camera
M
Beam
Block
0
r1
BS
o
irr
/9
10
M
Glass
ND Filter
Figure 3. Turning mirrors added to setup.
D. Understanding the Functions of the Beams
We are now ready to add the important 50/50 beam splitter as shown in Figure 4. But first
let’s talk about the motivation for all the beams flying around. The initial 10%/90%
beamsplitter has generated two weak 'probe' beams, and you've seen that each of them,
passing through the cell to a photodetector, is a probe of the transmission of the cell. But the
stronger beam transmitted through the 10/90 beamsplitter has now been brought around to the
far side of the cell, and is ready to be sent through the cell, in the opposite direction of the
probe beams, and overlapping one of the two probe beams inside the cell. (You want to
overlap the beam going to Detector 1.) The stronger beam is called the 'pump' beam, and
what it 'pumps' is the atoms being probed by only one of the two probe beams. Because we
are using a 50/50 beamsplitter, only half the pump beam is sent through the cell, and only half
of the probe beam gets through to the photodetector, PD1. The important function of the
50/50 BS, however, is to create the desired anti-parallelism of the pump beam and one of the
probe beams.
3 - 21
Rev 2.0 11/09
There are two fine points to observe in Figure 4. First note the upper adjustment knob on the
50/50 BS mount is on the side such that the probe beam can pass through the mount. You
should also observe that the mount is placed such that the beam going to Detector 1 passes
through the 50/50 beam splitter, but the beam going to Detector 2 misses both the beam
splitter and the edge of the mount that is holding the BS. With the 50/50 beam splitter in
place, we are ready to align the strong pump beam so that it is anti-parallel to the weak probe
beam going to detector 1. (You may want to read the appendix that has a short discussion of
the algorithm used to position a beam in space.)
Remove the glass ND filter from the beam path. This will make it easier to see the two
beams. Use the IR viewing card to observe the beams at position 1 which is right before the
probe beam goes through the 50/50 BS. The IR viewing card has a circular hole on its
backside so that you can observe beams from both directions.
Use the adjustment screws on Mirror 1 or 2 to overlap the two beam spots at position 1.
Detector 2
to (+) INPUT
Detector 1
to (-) INPUT
Photodiode
Detector
Photodiode
Detector
M
BS
irr
0
or
2
/5
50
IR viewing card
position 1
Rb Cell
CCD
Camera
Over lap these
two beams
IR viewing card
position 2
/9
10
M
0
or
1
BS
irr
Glass
ND Filter
Figure 4. Aligning pump and probe beams.
3 - 22
Rev 2.0 11/09
Now, move the IR viewing card to position 2 (between the Rb cell and the 10/90 BS). Use
the adjustment screws on the 50/50 BS mount to overlap the two beams at this position. It is
very likely that the strong pump beam will not be visible at position 2 initially. You may have
to loosen the screw that secures the post on the 50/50 BS and rotate it till you can find the
beam.
If all else fails and you cannot get the beams to overlap easily, you can temporarily move the
Rb cell and magnet off to the side so that you can trace the pump beam path from the 50/50
BS. Once the beams are overlapped at position 2, move back to position 1 and check the
beams. Again use the mirrors to overlap the beams here. After a few iterations, you should
be able to get the pump beam and one of the probe beams overlapping in space and antiparallel in direction.
Now replace the glass ND filter (and the Rb cell, if you removed it) into the beam path. Look
at the absorption signal on the oscilloscope. Expand the scale so that you can observe the two
large absorption features. If your beams are close to being aligned, you will start to see some
sharp spikes within the broad absorptions. See Figure 5. These “spikes” indicate that the
ability of the rubidium atoms to absorb photons from the probe beam has been diminished;
more light from the probe beam is actually reaching the detector. This is because atoms
which, in the past, would have absorbed the probe beam photons are already in the excited
state because they have absorbed photons from the “pump” beam. You may now try to
maximize the size of these spikes by tweaking the adjustment screws on the mirrors and the
50/50 BS.
(a)
(b)
Figure 5. Observation of SAS features
(a) Beams are partially overlapped and some SAS signal is visible
(b) Signal after tweaking of mirrors and 50/50 beamsplitter.
If you are “too” good at this alignment, the two beams may be so perfectly anti-parallel that
the strong pump beam comes through the cell and, bouncing off the 10/90, is reflected back
into the laser. When this happens, the laser will no longer scan through the spectrum
continuously, but in a series of steps. You may observe a spectrum that looks like that shown
3 - 23
Rev 2.0 11/09
in Figure 6. This feedback is undesirable, but it does show that you have perfected the
alignment of the two beams. Now you can slightly misalign the two beams such that the
feedback does not corrupt the smooth scan of the laser.
You may have noticed that the Caltech lab notes show an opto-isolator right after the laser.
The opto-isolator will stop this feedback from getting into the laser, but it is not essential for
operation of the system. Another technique to reduce feedback is to put more ND filters in
the beam path. An added filter attenuates the reflected beam twice, once on the way out and
again on the return trip.
Figure 6. When the anti-parallelism is too close to perfect, there is feedback into the laser
that corrupts the frequency sweep. The ‘staircase’ appearance of the absorption
profile is the indication of this.
If you have set up the second photodetector, you will now be able to use an electronics “trick”
to isolate the SAS features. To preview this capability, send the two photodetector signals to
the two channels of an oscilloscope, and adjust things until you can see what's similar about
the two signals, and what's different. Now you are ready use the detector electronics section
of your electronics box to isolate that difference. (You will be subtracting out most of the
broad absorption signal.)
Put the signal from Detector 1 into the minus input and that from Detector 2 into the plus
input of the detector section of the electronics box. Attach the monitor output to the ‘scope.
Set the plus balance control to zero and the minus balance control to one and observe the
signal from Detector 1 on the ‘scope. Adjust the gain on Detector 1 so that you have a large
signal (several volts) but not so large as to saturate the detector (maximum signal less than 10
volts). Now, set the plus balance to one and the minus balance to zero and observe the signal
form Detector 2. It will be inverted, with negative voltage values. Again adjust the gain of
Detector 2 for a signal level that is comparable to that seen by Detector 1. Because the beam
going to Detector 2 is not attenuated by the 50/50 beam splitter, the gain needed on Detector 2
will be less than that of Detector 1. (Typically Detector 1 needs a gain setting of 1.0 MΩ and
Detector 2 a gain of 330 kΩ.)
3 - 24
Rev 2.0 11/09
Now set both balance knobs to one and then reduce the balance on the larger signal so that the
Doppler broadened background is removed. This subtraction is never perfect, so there will
always be some residual broad absorption signal remaining. You may now raise the gain
setting on the difference signal and bring the SAS spikes up to the volt level. You are now
ready to record some beautiful SAS traces.
(a)
Figure 7. SAS traces with background subtraction.
(a) Rb87 F=2 and Rb85 F=3
(b) Expanded view of Rb87 F=2.
(b)
It is interesting to study these signals as a function of the intensity in each of the beams. The
above traces are power broadened. To observe the narrowest linewidths, you will have to
work at very low optical power levels in both the pump and the probe beams. You can use
neutral-density filters to attenuate the beams. You will also need to darken your room to
minimize ambient light falling into your photodetectors.
3 - 25
Rev 2.0 11/09
IV. Aligning a Michelson Interferometer
Mirror 2
Photodiode
Detector
Mirror 1
50:50 Beam
Splitter
Pick-off portion
of strong beam
0
/9
10
BS
Figure 1: Overview of un-equal arm Michelson.
Find a spot on the table to lay out the Michelson interferometer (MI). You must use the filter
holder to hold the 50/50 Beam Splitter (BS). A mirror mount will not allow the beam to come
in and exit from all four directions. This complicates the alignment as one can only make
coarse adjustments of the 50/50 BS.
Keep Mirror 1 as close to the 50/50 BS as possible.
Use a wedged or flat piece of glass as the 10/90 BS to pick off a fraction of the laser beam.
Now look at Figure 2 below. We will use a business card with a hole punched in it to observe
the reflected beam from each of the mirrors. Adjust the business card and card holder so that
the incoming beam goes through the hole. Use the CCD camera to observe the beam reflected
from the mirror. Adjust the mirror to send the out going beam back through the same hole.
Do this for both mirrors as shown in Figure 2.
3 - 26
Rev 2.0 11/09
Mirror 1
D
C ra
C me
a
C
50:50 Beam
Splitter
D
C ra
C me
a
C
Business Card w/ hole
in Card Holder
Mirror 2
Mirror
50:50 Beam
Splitter
Business Card w/ hole
in Card Holder
Figure 2: Use a business card with a hole punched in it to roughly align two mirrors.
3 - 27
Rev 2.0 11/09
Mirror 1
Business Card
in Card Holder
Mirror 2
Mirror
Mirror
C CC
am D
er
a
50:50 Beam
Splitter
Mirror 1
Business Card
in Card Holder
Mirror 2
Mirror
Mirror
C CC
am D
er
a
50:50 Beam
Splitter
Figure 3 Iterative procedure to get two beam co-linear.
Now move the Card holder to the position shown in Figure 3. (Upper) You should see two
beams on the card, one from each mirror.
With the card holder close to the 50/50 BS adjust mirror 2 to make the two beams over lap.
Then move the card holder to a position far from the 50/50 BS (a few feet, 1/2 meter or so.)
Again, you should see two beams, now adjust mirror 1 to make the beams overlap.
Go back to the near position and repeat.
In a few iterations, you should start to see some fringes appear in the overlapped area of the
two beams.
You will not see any fringes if the laser is scanning its wavelength, so turn off the wavelength
scan during this part of the operation. (Set the Ramp Generator attenuator to zero.)
Once you see some fringes you can still repeat the above steps a few more times. If done
correctly the fringe spacing should become larger as the alignment approaches optimum.
Gently pushing on one of the mirror mounts should cause the fringe pattern to change.
Now put a photodiode in the beam and restart the laser scan. You should be able to see some
nice periodic modulations of signal from the photodiode.
I find that a contrast ratio of 10% is about the best I can do. (minimum intensity is 10 % of
maximum intensity.)
Other tips:
Remove the glass Neutral density filter from the laser beam when doing the alignment. This
will help make the beams easier to see. You will have to replace the filter when you want to
make scans.
Feed back from the interferometer can get back into the laser and corrupt the scan. If this is a
problem adding more attenuators after the glass neutral density filter will help.
3 - 28
Rev 2.0 11/09
V. Appendix – Making Beams Collinear
Two points define a line, iterative procedure to align a laser beam to a “line in space”. The
pictures are only for aligning in one dimension.
The process is shown in the Figures 1 – 4 below. The objective is to get the laser beam, the
narrow line, to be collinear to the “line in space” represented by the darker dashed line. The
angles have been exaggerated to make it easier to see what is going on.
1. With the viewing card near to mirror M2, adjust angle of mirror M1 until the laser beam is
intersecting with the desired “line in space”. See Diagrams 1 and 2.
2. Now, move the viewing card to a distance far away from M2, as shown in Diagram 3.
3. Adjust the angle of mirror M2 so that laser beam again intersects the “line in space”. You
will notice that this makes the alignment at the first position, near M2, off a bit.
4. Now move the viewing card back to position shown in Diagram 1 and repeat.
You will probably have to repeat the process several times to get the beam where you want it.
(That’s iterative for you!) The closer the viewing card is to M2, the faster this procedure
converges.
You might ask where the “line in space” that you are trying to match comes from. It could be
another laser beam or, perhaps, a desired beam path defined by two irises.
3 - 29
S.
B.
S.
B.
M
irr
or
0
/5
50
0
/5
50
M
irr
or
Rev 2.0 11/09
IR Viewing Card
Line to match
Line to match
Line to match
3.)
S.
B.
S.
B.
0
/5
50
M
irr
or
2.)
0
/5
50
M
irr
or
1.)
Line to match
IR Viewing Card
4.)
IR Viewing Card
IR Viewing Card
3 - 30